1. Field of the Invention
The present invention relates to a method for optical resolution of Corey lactone diols which are intermediates of prostaglandins useful as drugs.
2. Description of Prior Art
Prostaglandins are physiologically active substances and play a fundamental role in maintaining homeostasis of the living body so that prostaglandins are expected to be utilized as drugs in various fields. Among them, prostaglandins have already been provided for practical use in the fields of oxytocia (rapid parturition), improvement in peripheral blood stream, anti-ulcer, prevention of platelet agglutination, etc.
The most effective method for chemical synthesis of prostaglandins comprises using as a key intermediate Corey lactones (formula [A]) reported by Corey et al.: ##STR2## wherein R' and R" represent hydrogen atom or a hydroxy-protecting group (Reference publication: JACS, 92, 397 (1970), JACS, 93, 1490 (1971)).
Since many prostaglandins are synthesized still now via Corey lactones, Corey lactiones are compounds indispensable for synthesis of prostaglandins.
For synthesis of optically active Corey lactones, many methods are known. Among others, the following process is considered to be excellent.
1) Corey process . . . synthetic process comprising as key steps Diels-Alder reaction of cyclopentadiene derivatives and optical resolution of hydroxycarboxylic acid (Reference Publications: JACS, 92, 397 (1970), JACS, 93, 1490 (1971)).
2) Chinoin process . . . synthetic process comprising as a key step Prins reaction of optically active dicyclic lactones obtained by optical resolution (Reference Publications: Japanese Patent Application Laid-Open No. Sho 52-105163, Tetrahedron Letters (TL), 4639 (1976)).
3) Pfizer process . . . synthetic process comprising as key steps Prins reaction of norbornadiene and optical resolution of carboxylic acids (Reference Publications: Japanese Patent Application Laid-Open No. Sho 50-111074, JACS, 95, 7522 (1973)).
4) Asymmetric Diels-Alder process . . . synthetic process comprising as a key step asymmetric Diels-Alder reaction using an optically active pantholactone as an asymmetric source (Reference Publication: Japanese Patent Application Laid-open No. Sho 63-152339)).
The three processes 1) through 3) described above all require optical resolution and thus encounter problems that expensive agents for optical resolution should be used, operation for optical resolution such as salt formation, filtration, salt decomposition, recovery of agents used for optical resolution, etc. are complicated. The asymmetric Diels-Alder process 4) is excellent since Corey lactones can be produced in a high yield with high selectivity without optical resolution. After asymmetric reaction, however, the reaction steps for leading to Corey lactones are large in number. Therefore, the process was yet unsuitable as a process in an industrial scale.
For the foregoing reasons, a simpler method for optical resolution of Corey lactone esters has been desired.